Steam cracking, also referred to as pyrolysis, has long been used to crack various hydrocarbon feedstocks into olefins, preferably light olefins such as ethylene, propylene, and butenes. Conventional steam cracking utilizes a pyrolysis furnace which has two main sections: a convection section and a radiant section. The hydrocarbon feedstock typically enters the convection section of the furnace as a liquid (except for light feedstocks which enter as a vapor) wherein it is typically heated and vaporized by indirect contact with hot flue gas from the radiant section and by direct contact with steam. The vaporized feedstock and steam mixture is then introduced into the radiant section where the cracking takes place. The resulting products, including olefins, leave the pyrolysis furnace for further downstream processing, including quenching.
Conventional steam cracking systems have been effective for cracking high-quality feedstocks such as gas oil and naphtha. However, steam cracking economics sometimes favor cracking low cost heavy feedstock such as, by way of non-limiting examples, crude oil and atmospheric resid, also known as atmospheric pipestill bottoms. Crude oil and atmospheric resid contain high molecular weight, non-volatile components with boiling points in excess of 590° C. (1100° F.). The non-volatile, heavy ends of these feedstocks lay down as coke in the convection section of conventional pyrolysis furnaces. Only very low levels of non-volatiles can be tolerated in the convection section downstream of the point where the lighter components have fully vaporized. Additionally, some naphthas are contaminated with crude oil during transport. Conventional pyrolysis furnaces do not have the flexibility to process resids, crudes, or many resid or crude contaminated gas oils or naphthas, which contain a large fraction of heavy non-volatile hydrocarbons.
The present inventors have recognized that in using a flash to separate heavy non-volatile hydrocarbons from the lighter volatile hydrocarbons which can be cracked in the pyrolysis furnace, it is important to maximize the non-volatile hydrocarbon removal efficiency. Otherwise, heavy, coke-forming non-volatile hydrocarbons could be entrained in the vapor phase and carried overhead into the furnace creating coking problems in the convection section.
Additionally, during transport some naphthas are contaminated with heavy crude oil containing non-volatile components. Conventional pyrolysis furnaces do not have the flexibility to process residues, crudes, or many residue or crude contaminated gas oils or naphthas which are contaminated with non-volatile components.
To address coking problems, U.S. Pat. No. 3,617,493, which is incorporated herein by reference, discloses the use of an external vaporization drum for the crude oil feed and discloses the use of a first flash to remove naphtha as vapor and a second flash to remove vapors with a boiling point between 230 and 590° C. (450 and 1100° F.). The vapors are cracked in the pyrolysis furnace into olefins and the separated liquids from the two flash tanks are removed, stripped with steam, and used as fuel.
U.S. Pat. No. 3,718,709, which is incorporated herein by reference, discloses a process to minimize coke deposition. It describes preheating of heavy feedstock inside or outside a pyrolysis furnace to vaporize about 50% of the heavy feedstock with superheated steam and the removal of the residual, separated liquid. The vaporized hydrocarbons, which contain mostly light volatile hydrocarbons, are subjected to cracking.
U.S. Pat. No. 5,190,634, which is incorporated herein by reference, discloses a process for inhibiting coke formation in a furnace by preheating the feedstock in the presence of a small, critical amount of hydrogen in the convection section. The presence of hydrogen in the convection section inhibits the polymerization reaction of the hydrocarbons thereby inhibiting coke formation.
U.S. Pat. No. 5,580,443, which is incorporated herein by reference, discloses a process wherein the feedstock is first preheated and then withdrawn from a preheater in the convection section of the pyrolysis furnace. This preheated feedstock is then mixed with a predetermined amount of steam (the dilution steam) and is then introduced into a gas-liquid separator to separate and remove a required proportion of the non-volatiles as liquid from the separator. The separated vapor from the gas-liquid separator is returned to the pyrolysis furnace for heating and cracking.
Co-pending U.S. application Ser. No. 10/188461 filed Jul. 3, 2002, Patent Application Publication US 2004/0004022 A1, published Jan. 8, 2004, which is incorporated herein by reference, describes an advantageously controlled process to optimize the cracking of volatile hydrocarbons contained in the heavy hydrocarbon feedstocks and to reduce and avoid coking problems. It provides a method to maintain a relatively constant ratio of vapor to liquid leaving the flash by maintaining a relatively constant temperature of the stream entering the flash. More specifically, the constant temperature of the flash stream is maintained by automatically adjusting the amount of a fluid stream mixed with the heavy hydrocarbon feedstock prior to the flash. The fluid can be water.
U.S. Patent Application Ser. No. 60/555282, filed Mar. 22, 2004, which is incorporated herein by reference, describes a process for cracking heavy hydrocarbon feedstock which mixes heavy hydrocarbon feedstock with a fluid, e.g., hydrocarbon or water, to form a mixture stream which is flashed to form a vapor phase and a liquid phase, the vapor phase being subsequently cracked to provide olefins, and the product effluent cooled in a transfer line exchanger, wherein the amount of fluid mixed with the feedstock is varied in accordance with a selected operating parameter of the process, e.g., temperature of the mixture stream before the mixture stream is flashed.
Co-pending U.S. application Ser. No. 10/189618 filed Jul. 3, 2002, Patent Application Publication US 2004/0004028 A1, published Jan. 8, 2004, which is incorporated herein by reference, describes an advantageously controlled process to increase the non-volatile removal efficiency in a flash drum in the steam cracking system wherein gas flow from the convection section is converted from mist flow to annular flows before entering the flash drum to increase the removal efficiency by subjecting the gas flow first to an expander and then to bends, forcing the flow to change direction. This coalesces fine liquid droplets from the mist.
It has been found that in the convection section of a steam cracking pyrolysis furnace, a minimum gas flow is required in the piping to achieve good heat transfer and to maintain a film temperature low enough to reduce coking. Typically, a minimum gas flow velocity of about 30 m/sec (100 ft/sec) has been found to be desirable.
When using a vapor/liquid separation apparatus such as a flash drum to separate the lighter volatile hydrocarbon as vapor phase from the heavy non-volatile hydrocarbon as liquid phase, the flash stream entering the flash drum usually comprises a vapor phase with liquid (the non-volatile hydrocarbon components) entrained as fine droplets. Therefore, the flash stream is two-phase flow. At the flow velocities required to maintain the required boundary layer film temperature in the piping inside the convection section, this two-phase flow is in a “mist flow” regime. In this regime, fine droplets comprising non-volatile heavy hydrocarbons are entrained in the vapor phase, which is the volatile hydrocarbons and, optionally, steam. The two-phase mist flow presents operational problems in the flash drum because at these high gas flow velocities the fine droplets comprising non-volatile hydrocarbons do not coalesce and, therefore, cannot be efficiently removed as liquid phase from the flash drum. It was found that, at a gas flow of 30 m/s (100 feet/second) velocity, the flash drum can only remove heavy non-volatile hydrocarbons at a low efficiency, e.g., about 73%.
The present invention provides an apparatus and process for the effective removal of non-volatile hydrocarbon liquid from the volatile hydrocarbon vapor in the flash drum. The present invention provides an apparatus and process that significantly enhance the separation of non-volatile and volatile hydrocarbons in the flash drum.